The preparation of aromatic and aliphatic-aromatic carbonic acid esters (carbonates) by transesterification starting from aliphatic carbonic acid esters and aromatic hydroxy compounds has been described. Such preparative methods are equilibrium reactions in which the position of equilibrium is shifted almost completely towards the aliphatically substituted carbonates. It is therefore comparatively simple to prepare aliphatic carbonates from aromatic carbonates and alcohols. However, in order to carry out the reactions in the opposite direction, in the direction towards aromatic carbonates, it is necessary to shift the equilibrium, which is positioned very unfavorably, to the side of the aromatic carbonates, it being necessary to use not only very active catalysts but also suitable procedures.
Carrying out such equilibrium reactions in columns and thus advantageously shifting them towards the formation of the desired product has been described, for example, in: U. Block, Chem.-Ing. Techn. 49, 151 (1977); DE-OS 38 09 417; B. Sehleper, B. Gutsche, J. Wnuck and L. Jeromin, Chem.-Ing.-Techn. 62, 226 (1990); Ullmanns Encyclopädie der techn. Chemie, 4th Edition, Vol. 3; p. 375 ff. (1973).
In the described processes, the transesterification is therefore also preferably carried out continuously as a countercurrent transesterification in one or more reaction columns.
EP-A 0 461 274 describes a continuous transesterification process for the preparation of aromatic carbonates in one or in a plurality of multistage columns connected in series, wherein dialkyl carbonates or alkylaryl carbonates are reacted with phenols and the readily volatile products, namely the reaction alcohols and dialkyl carbonates, are removed at the head (top) of the columns and the high-boiling products, such as, for example, diaryl carbonates, are removed at the bottom of the columns. However, there is no direction as to the manner or extent to which heat produced in this process can be further used.
DE-A 42 26 756 describes a two-stage process for the preparation of diaryl carbonates by transesterification of a dialkyl carbonate with an aromatic hydroxy compound, in which the corresponding alkylaryl carbonate is first formed from the starting materials in a first stage and the diaryl carbonate is formed in a second stage. The information given in the process description is limited to the reaction conditions, the catalyst used and the construction of the reaction columns. No information is given, however, regarding the manner or extent to which heat produced in this process can be further used.
DE-A 42 26 755 describes a process for the preparation of diaryl carbonates in two reaction columns which are coupled with one another in terms of energy and materials, wherein an aromatic hydroxy compound and a dialkyl carbonate are reacted in the first stage, and the alkylaryl carbonate formed thereby is converted into the diaryl carbonate in the second stage either by transesterification with the aromatic hydroxy compound or by disproportionation. However, a problem with this process is that, owing to the integration of the process in terms of materials and energy, the reaction conditions for the formation of the alkylaryl or diaryl carbonate cannot be chosen optimally because they are determined by the almost identical pressure prevailing in the two steps.
EP-A 781 760 describes a continuous process for the preparation of aromatic carbonates by reacting a dialkyl carbonate with an aromatic hydroxy compound in the presence of a catalyst and continuously removing the aromatic carbonate formed in the reaction, the alcoholic secondary products, the dialkyl carbonate and the aromatic hydroxy compound, the dialkyl carbonate and the aromatic hydroxy compound being fed back into the reaction again. Although the described process steps are effective as regards the reaction procedure in terms of a high space-time yield and as regards working-up in terms of an efficient a separating sequence as possible, the process does not exhibit any possibilities for integration of the reaction and the working-up steps in terms of energy.
WO-A 2006/001256 describes a process in which an aromatic hydroxy compound is reacted with a dialkyl carbonate in the presence of a catalyst, as well as a technical device suitable therefor. Here too, no reference points are given for energy integration.
Without appropriately efficient energy integration, the energy consumption of the processes described hereinbefore is known to be high, which in turn raises questions about the advantageousness of the phosgene-free preparation of aryl carbonates from an ecological and economic point of view.
WO-A 2004/016577 describes a process for the preparation of aromatic carbonates from dialkyl carbonate and an aromatic hydroxy compound in the presence of a catalyst in a plurality of separate, series-connected reaction zones of a reactor arrangement, wherein the heat of condensation that is formed in the condensation of the vapor stream of the last reaction zone is used to heat the liquid stream introduced into the first reaction zone. However, this process has the disadvantage that the reactor arrangement is complex. In addition, the energy integration of this process is worthy of improvement.
JP-A 2002-020351 describes a discontinuous process for the preparation of diaryl carbonate, from which heat can be used for the production of steam. Disadvantages of this process are, however, that it is carried out discontinuously and the reactor arrangement used for the reaction with a distillation column mounted on top. However, a particular disadvantage of this process is that it is carried out discontinuously.